The encoding function can be carried out by one or more polarized magnetic marks which form an encoding zone whose displacement is detected by a magnetic field detector disposed facing this encoding zone. For this type of encoder, it is known, in particular from the document FR-A 2 777 060, to produce the mark or marks of the encoding zone using a ferrite filled elastomer layer, that is to say a polymer of the elastomer type containing a high proportion of magnetic particles such as, for example, ferrite powder, which is adherized during its moulding onto the sleeve of the encoder. Such a solution is practical to use because of the formation of the magnetic layer and of the polarized marks, and the adherization of this layer to the sleeve during the moulding process.
However, the magnetic field created by the ferrite filled elastomer does not reach a sufficient intensity for certain applications, in particular when the air-gap between the encoding zone reaches a value that is too large because of the space available for the magnetic field sensor or because of the variations in the air-gap during the functioning of the device. In order to improve the intensity of the magnetic field created by a ferrite filled elastomer, it is known to increase the proportion of magnetic particles. However, the densities of magnetic particles henceforth achieved make the ferrite filled elastomer too fragile and crumbly for certain applications.